Instead, European leaders had to listen to Trump saying that "23 of the 28 member nations are still not paying what they should be paying and what they are supposed to be paying", and that they owe "massive amounts" from past years - a misstatement of NATO's spending targets, which guide nations' own domestic spending decisions. Far from robustly reaffirming Nato's mutual defence commitment in the way many members hoped he would, Mr Trump repeated his complaint that the United States was shouldering an unfair burden.

Last year, Trump threatened to abandon US allies in Europe if they did not spend enough on defense, comments that were particularly unnerving for the ex-Soviet Baltic states on Russia's border which fear Moscow might try a repeat of its 2014 annexation of Ukraine's Crimea.

Trump's discomfort has been particularly obvious in comparison to European leaders, who move easily in a pack.

If NATO allies were nervous about the United States' commitment to Europe's security before, they must be fuming now. "2 percent is the bare minimum for confronting today's very real and very vicious threats", Trump said.

And while the liberal mainstream media have tripped all over themselves to dispute President Trump's claim, they did not disagree with that claim when it was made by President Obama in his April 2016 interview with The Atlantic.

A senior administration official said that no one should "read into" Trump's remarks any lack of U.S. commitment to the alliance's collective defense obligations. Only five - the United States, Greece, Great Britain, Estonia, and Poland - are living up to that agreement with the United States paying nearly twice as much percentage-wise at about 3.7 percent in 2016.

Reaction to the summit and Trump's visit was swift, with Charles Michel, the Prime Minister of Belgium whose country has hosted North Atlantic Treaty Organisation for over 50 years, warning, "Security today requires much more than the traditional military tools. This is why we will indeed make our contribution to security and solidarity in the common alliance", she said. His initial interest in warmer relations with Russian Federation put off allies on the Continent, who have cast a wary eye on Putin's activities from the Baltics to the Balkans.

Indeed, NATO Secretary-General Jens Stoltenberg underscored to journalists Thursday that NATO's European members have halted a long-term slide in military spending and registered an overall increase in defense budgets previous year.

Trump's concern over military expenditures is legitimate.

Having said that, Trump's words miss the point. He might just as well have said, "Do widzenia".

"The mood of Article 5, the idea that we are all in this together, is not the mood he conveyed", said Jon Alterman, a senior vice president at the Center for Strategic and International Studies in Washington. Or not. Again, that question is left hanging.

In a sign of growing frustration felt by EU leaders, European Parliament President Antonio Tajani, speaking after meeting Trump in Brussels, defended European defense spending, insisting: "Europe is taking on greater responsibility on defense".

"You could argue that the rhetorical style of the new president is not the most agreeable for European ears", he says.

The day will feature a welcoming ceremony and concert at the remains of an ancient Greek temple, as well as a relentless number of meetings, many of which White House aides are hoping to keep short in order to keep Trump's attention.

In dedicating the 9/11 Article 5 memorial, the president was "sending a strong signal" of his commitment to Nato, Mr Stoltenberg said.